In an engine system, especially in a compression-ignition engine, such as a diesel engine, injectors are used in engine cylinders and exhaust gas processing systems. In both applications, the main functions of the injector include controlling a flow rate of a working fluid and atomizing the working fluid.
In applications of in-cylinder fuel injections, a variety of methods can be used for controlling flow rate. Among them, a pre-metering method, in which a fluid is metered and then injected, and a common-rail method, in which a flow rate of a fluid is controlled by adjusting opening time of an injector in a repeating cycle, are commonly used. Fuel atomization in these applications is normally achieved with high injection pressure.
In exhaust gas processing systems, however, injection pressure is limited. For example, in a DPF (Diesel Particulate Filter) system, especially in a DPF system of an engine with a common rail fuel system, to regenerate the DPF, an external doser can be used for delivering fuel into a combustion device, which normally includes a DOC or a fuel burner. In the external doser, typically a lifting pump in the engine fuel system, which is primarily used for delivering low pressure diesel fuel to high pressure pumps, is employed for providing fuel to an injector, though which fuel delivery rate is controlled. The lifting pressure is much lower than the in-cylinder injection pressure. In a SCR (Selective Catalytic Reduction) system, DEF (Diesel Exhaust Fluid), which is a 32.5% wt urea solution, needs to be delivered to exhaust gas to reduce NOx therein. Limited to penetration distance, cost, and device size, normally only low pressure pump (lower than 10 bars) are used in DEF delivery.
Though injection pressure is low, exhaust gas processing systems are sensitive to working fluid atomization. In a DPF system, poor atomization causes delay in fuel oxidation in catalyst and fuel carbonization (coking), which may block doser nozzles and deteriorate temperature control performance, while in a SCR system, large droplet of DEF lowers deNOx efficiency and increases chances of urea crystallization, which may block injectors, catalysts, and even exhaust passages.
To obtain good atomization at low injection pressure, a variety of technologies can be used. One of the most commonly used technologies is using compressed air to assist working fluid injection. In this technology, the working fluid metered by a metering pump or a metering injector is mixed with compressed air, and the result mixture is then delivered to exhaust gas. However, in the air-assisted technology, compressed air also goes into exhaust gas with the working fluid. The compressed air lowers exhaust temperature, which is critical to reactions in the exhaust gas processing system, resulting in evaporation issues, poor thermolysis, and higher energy cost, and evaporates working fluid in the mixer in which the working fluid mixes with the compressed air, causing deposit and crystallization therein. Additionally, compressed air in the mixer varies pressure drop across the injector when a metering injector is used, introducing errors in flow rate control.
Another technology is using an atomization means together with an injector, as disclosed in the U.S. Pat. No. 8,047,452, and U.S. Pat. No. 6,279,603, in which an atomization device is used for creating small DEF droplets. In this technology, the working fluid has to cycle through the injector downstream from a possible pressure sensor, sensing values obtained from which is used for compensating flow rate control. Cycling the working fluid through the injector carries heat away therefrom, however, heating working fluid may cause fluid quality issues, since some working fluid deteriorates at high temperature, for example, deterioration of DEF starts to accelerate at a temperature higher than 50° C., while working fluid cycling downstream from the pressure sensor causes variations in pressure difference across the injector nozzle undetected, resulting in errors in flow rate control.
To solve the problems mentioned above, a primary object of the present invention is to provide a working fluid injector with which a good atomization can be achieved at low injection pressure without using compressed air.
A further objective of the present invention is to provide a working fluid injector pressure drop across which can be detected accurately through a pressure sensor upstream, thereby accurate flow rate control can be obtained.
Another objective of the present invention is to provide a working fluid injector that is able to create self-spinning of working fluid droplets to decrease spray angle and penetration distance, and improve atomization at low injection pressure.